1. Field of the Invention
The present invention relates to signal demultiplexor circuits, in particular, to integrated signal demultiplexor circuits for separating signal components having both different magnitudes and frequencies.
2. Description of the Related Art
As integrated circuits have become more complex and sophisticated by including more functions and capabilities, an increasingly common and significant problem is that of sufficient available terminals, or pins, through which to convey the necessary interface signals, whether input or output. Of particular interest, for purposes of the present invention, are those circuits used for providing the signal processing and control functions for video circuits, such as for video monitories and displays.
In particular, two common interface signals which in recent years have become combined, or multiplexed, into one signal are the horizontal clamp and vertical synchronization signals used for controlling the horizontal clamp circuitry and synchronizing the vertical scanning circuitry, respectively, within a video monitor or display, such as those used for computers. As is well known in the art, the horizontal clamp signal has a typical period of 30 kilohertz (khz) with a typical asserted pulse width of three microseconds (usec). As is also well known, the vertical synchronization signal has a typical period of 70 khz, with a typical asserted pulse width of 500 usec. When multiplexed, the combined signal, sometimes referred to as a “sand castle” signal because of the shape of the waveform, includes a horizontal clamp signal component summed with a vertical synchronization signal component. The vertical synchronization signal component is approximately one half the amplitude of the horizontal clamp signal component, thereby displaying a tri-level signal characteristic. By doing this, only one interface signal path is required instead of two, while the constituent signal components are easily demultiplexed, or separated.
However, it is this signal demultiplexing, or separating, where problems arise, generally in the form of undesirable time delays between the demultiplexed signal components. Because of the multiplexed nature of the signal, typical demultiplexing techniques involve the use of filtering. With the relatively significant frequency difference between the two signal components, filtering is a simple and otherwise desirable technique for separating the signal components. However, filtering necessarily involves the use of capacitance, and as a result, undesirable time delays are often introduced.
Accordingly, it would be desirable to have a technique for demultiplexing combined signals of different frequencies with minimal, if any, use of filtering, particularly in an integrated circuit environment where the implementation of capacitors is difficult and expensive in terms of die surface area.